Onboard floating drilling installations, like drillships or semisubmersible drilling rigs, are maritime vessels which have been fitted with drilling equipment. Onboard floating drilling installations are normally used for exploratory drilling of new oil or gas wells in deep water but can also be used for scientific drilling. They are able to drill in water depths up to 3500-4000 m. Most of the onboard floating drilling installations are outfitted with a dynamic positioning system to maintain position over the well.
A dynamic positioning system is a computer controlled system to automatically maintain an onboard floating drilling installation's position and heading by using own propellers and thrusters. Position reference sensors, combined with motion sensors, wind sensors and gyro compasses, provide information to the computer of the dynamic positioning system pertaining to the onboard floating drilling installation's position and the magnitude and direction of environmental forces affecting its position.
The computer program working in the dynamic positioning system contains a mathematical model of the onboard floating drilling installation that includes information pertaining to the wind and current drag of the onboard floating drilling installation and the location of the thrusters of the onboard floating drilling installation. This knowledge, combined with the sensor information, allows the computer to calculate the required steering angle and thruster output for each thruster. This allows operations at sea where mooring or anchoring is not feasible due to deep water, congestions of pipelines or drilling equipment on the sea bottom or other problems.
Dynamic positioning may either be absolute in that the position of the onboard floating drilling installation is locked to a fixed point over the sea bottom, or relative to a moving object like a ship or another underwater vehicle. The onboard floating drilling installation can also be positioned at an advantageous angle towards wind, waves and flow of water.
Such onboard floating drilling installations comprises draw works for reeling out and reeling in a drilling line, whereby the draw works may comprise at least one drive. Furthermore onboard floating drilling installations comprise several propellers or thrusters for dynamic positioning the onboard floating drilling installation. Further, onboard floating drilling installations comprise one or more generators for producing electric current to operate the thrusters and the draw works.
Onboard floating oil drilling installations such as semisubmersible drilling rigs or drillships trend towards electrically operated draw-works with active heave compensation. This is used in order to keep a constant pressure on the drill bit during the drilling. This generates a need for a big oscillating load at the frequency of the sea waves.
Until now the problem has been solved by pulling the power from the onboard generators and dissipating the braking energy in braking resistors. This dissipates large amounts of energy in the braking resistors. Furthermore, this results in a varying load for the generators and thus poor fuel efficiency and also possibilities for an unstable voltage and frequency.